As the aircraft was not fitted with a fire indicator, the presence of the instructor permitted fast detection of the fire and interruption of the take-off. No trace of the fuel filter was found. The absence of the filter, which should have been connected to the auxiliary fuel pump which fell to the ground during the fire, coupled with the observed fuel leak, would seem to indicate that the filter was consumed by the fire, and that it was improperly installed. The fitting that connected the filter to the pump was not tight enough to prevent the possibility of a fuel leak. The pump and the filter should have been held in place by two retaining bolts. As one bolt was missing and no part of it was found, it is impossible to determine whether the bolt broke off or came undone on a previous flight, or how long it had been missing. Further, as the auxiliary fuel pump was fastened to the structure by only one bolt instead of two, the weakness of the installation of the pump and the filter, which connects to the pump at the fitting, might have contributed to causing a fuel leak. The considerable damage in this area of the aircraft, however, made it impossible to determine exactly what factor initiated the fire in the fuel. It was difficult for the pilot to stop the aircraft on the runway itself because the runway is only 2,000 feet long.Analysis As the aircraft was not fitted with a fire indicator, the presence of the instructor permitted fast detection of the fire and interruption of the take-off. No trace of the fuel filter was found. The absence of the filter, which should have been connected to the auxiliary fuel pump which fell to the ground during the fire, coupled with the observed fuel leak, would seem to indicate that the filter was consumed by the fire, and that it was improperly installed. The fitting that connected the filter to the pump was not tight enough to prevent the possibility of a fuel leak. The pump and the filter should have been held in place by two retaining bolts. As one bolt was missing and no part of it was found, it is impossible to determine whether the bolt broke off or came undone on a previous flight, or how long it had been missing. Further, as the auxiliary fuel pump was fastened to the structure by only one bolt instead of two, the weakness of the installation of the pump and the filter, which connects to the pump at the fitting, might have contributed to causing a fuel leak. The considerable damage in this area of the aircraft, however, made it impossible to determine exactly what factor initiated the fire in the fuel. It was difficult for the pilot to stop the aircraft on the runway itself because the runway is only 2,000 feet long. The fire broke out at the rear of the right nacelle in the area occupied by the filter, the auxiliary fuel pump, and the fuel selector. The auxiliary fuel pump had only one retaining bolt holding it to the structure instead of two. The fitting attaching the fuel filter to the auxiliary fuel pump inlet was not fully tightened and it was possible for fuel to leak. The fuel filter was consumed by the fire, and no debris was found.Findings The fire broke out at the rear of the right nacelle in the area occupied by the filter, the auxiliary fuel pump, and the fuel selector. The auxiliary fuel pump had only one retaining bolt holding it to the structure instead of two. The fitting attaching the fuel filter to the auxiliary fuel pump inlet was not fully tightened and it was possible for fuel to leak. The fuel filter was consumed by the fire, and no debris was found. A fuel leak in the connection between the auxiliary fuel pump and the fuel filter allowed a fire to start in the right nacelle during take-off. The absence of one of the fastening bolts from the auxiliary fuel pump and an inadequately tightened fitting are factors that may have contributed to the fuel leak.Causes and Contributing Factors A fuel leak in the connection between the auxiliary fuel pump and the fuel filter allowed a fire to start in the right nacelle during take-off. The absence of one of the fastening bolts from the auxiliary fuel pump and an inadequately tightened fitting are factors that may have contributed to the fuel leak.